Oil palm trunk core (OPTC) is the biomass left over from plywood industry that utilises oil palm trunk as feedstock. This research aims to investigate the fuel and ash properties of biochar converted from OPTC biomass through microwave-assisted carbonisation process. The raw OPTC contains high moisture (~64.3%) with low fixed carbon (~6.4%) and calorific value (4.6 MJ kg-1). Microwave-assisted carbonisation of OPTC was investigated under several sizes, temperature and microwave power variables. The results showed that biochar yield decreased with the increasing carbonisation temperature. The optimum conditions yielding 8.4% biochar was obtained at 350oC, microwave power 800 W and 6 cm cube. Compared to raw OPTC, biochar has higher fixed carbon (~20-64%) and about four times higher calorific value than the raw OPTC. In general, all of the biochars produced in this study showed higher alkali index and total alkali values than those obtained from raw OPTC. Feedstock size seemed to affect biochar ash properties in this study. Biochar obtained from 6 cm cube tend to have lower slagging and fouling tendency than those produced with 4 cm cube.